1. Field of the Invention
The present invention relates to optical fiber cable constructions and to methods and apparatus for fabricating the same.
2. Description of the Prior Art
Conventional optical fiber cables typically utilize the construction shown in cross-section in FIG. 1. As shown in FIG. 1, a plurality of fiber units 2, each consisting of a plurality of optical fibers 1, are gathered and stranded around a core member 3. Core member 3 is usually fabricated from a material that exhibits a high tensile strength. An outer roll or sheath 4 made of plastic material or the like is typically formed over the outer surfaces of fiber units 2 so as to form an outer covering. It should be noted that this construction inherently subjects the optical fibers 1 to substantial lateral or compression forces.
The construction of FIG. 1 exhibits a low ratio of optical fibers per cross-sectional unit area because of the dead spaces between adjacent cable units 2 and between adjacent cable units 2 and the outer sheath 4. In order to increase the ratio of optical fibers per cross-sectional unit area, other conventional optical fiber cable structures include additional optical fibers 1 in the above-described dead spaces. However, these conventional optical fiber cable structures inherently subject the optical fibers 1 in the cable units 2 as well as the optical fibers 1 in the dead spaces to substantial lateral or compression forces.
As is known in the art, lateral or compression forces have a deleterious effect on the optical performance and, thus, the transmission characteristics of the optical fibers. Specifically, such lateral or compression forces substantially increase transmission losses and substantially change the transmission pass band of the optical fibers.
One conventional approach directed at reducing the lateral or compression forces applied to the optical fibers in an optical fiber cable utilizes a rigid or semi-rigid support member disposed within the outer covering of the cable. The support member typically is fabricated using a plastic material or the like, and is provided with a plurality of extruded ribs which extend radially outwardly from the center-line of the cable and thus define a plurality of compartments which extend longitudinally in a substantially parallel fashion along the centerline of the cable. The optical fibers are accomodated into loosely-fitting fashion in the compartments. Because of the loose fitting that is provided by this construction, the optical fibers are not subjected to substantial lateral or compression forces, and, thus, do not exhibit increased transmission losses and changed transmission band width caused by such forces, as described above.
The use of the rigid or semi-rigid support member, however, results in several major deficiencies. For example, such a support member is difficult and expensive to fabricate and also makes the insertion of the optical fibers into the compartments very difficult, especially when an optical cable having a substantial length is involved. In addition, the support member must be specially fabricated when a different number of ribs, and, thus, compartments are desired, which inherently increases fabrication cost and manufacturing time. Furthermore, the use of a support member inherently produces an optical cable having a rigid or a semi-rigid characteristic, which often makes it difficult to install the optical cable. In this connection, it should be noted that the rigid or semi-rigid characteristic also makes it difficult to assemble the optical cable at a location different from the installation location because of the required transportation of the fabricated cable. Splicing of optical fibers is a complicated and expensive operation that inherently increases transmission loss and changes transmission bandwidth. Thus, it is desirable to be able economically to manufacture and install the optical fiber cable without having to provide any undesired cable splices.